                Patent-Related Reference 1: JP-A-11-295255        Patent-Related Reference 2: JP-A-2003-161713        
In the patent-related reference 1, it is mentioned that various gas sensors having different gas response properties can be obtained by providing two comb-shaped electrodes on a glass substrate, and by additionally forming thereon a sensitive film made from two kinds of mixed electroconductive polymers.
Further, mentioned in the patent-related reference 2 is an ammonia gas sensor including: an electrically insulative ceramic substrate; two comb-shaped electrodes formed thereon; and an ammonia sensitive layered film formed to extend over the two comb-shaped electrodes.
In each of the patent-related references, two pieces of metal are worked into comb shapes, respectively, and placed on one side of the associated rectangular parallelepiped substrate. Such a sensor has a limitation of downsizing while keeping a performance, thereby problematically making it difficult to mount the sensor on a readily recognizable portion such as a gas piping.